From a viewpoint of natural environmental conservation, there has recently been demand for biodegradable polymers decomposable in natural environment and moldings formed from biodegradable polymers, and biodegradable polymers such as aliphatic polyesters are actively studied. Particularly, lactic acid-based polymers have substantially high melting points ranging from 140 to 180° C. and excellent transparency, and hence much expectations are placed on these polymers as packaging materials and for moldings benefited from their transparency.
However, among containers obtained from lactic acid-based polymers by injection molding or the like, some have excellent mechanical properties but low heat resistance, and some are poor in both heat resistance and mechanical properties. When such containers are used as packaging containers, for example, hot water or microwave cannot be applied and hence their use is limited.
In order to obtain heat-resistant moldings, it has been required to highly crystallize the resin by prolonging die-cooling time during molding process or by annealing the moldings after molding. However, these methods have drawbacks: the prolonged cooling in molding process is impractical and likely to cause insufficient crystallization, while the crystallization by annealing after molding readily deforms the moldings during the crystallization.
As a method to increase crystallization rates of resins, for example, Patent Document 1 describes a method of adding fine powder of a wholly aromatic polyester mainly composed of terephthalic acid unit and resorcinol unit as a nucleating agent for promoting crystallization of polyethylene terephthalate (PET). Such addition of a nucleating agent is generally known as a method to accelerate crystallization of resins.
Patent Documents 2 to 10 describe addition of additives such as nucleating agents to biodegradable polymers.
Patent Document 2 discloses materials for plastic containers in which 10 to 40% by weight of calcium carbonate or talc with an average particle diameter of 20 μm or less is mixed with biodegradable plastics such as 3-hydroxybutyrate/3-hydroxyvalerate copolymer, polycaprolactone, or polylactic acid. In this technique, however, a large amount of inorganic filler was added to accelerate degradation of the biodegradable plastics discarded, not to improve heat resistance of the moldings through crystallization of the biodegradable polymers.
Patent Document 3 describes addition of an inorganic compound such as silica or kaolinite as a filler to thermoplastic lactide plastics can modify the properties in hardness, strength, and temperature resistance. According to Example therein, when 5% by weight of calcium lactate was added as a nucleating agent to L-/DL-lactide copolymer and the mixture was blended by a heating roll at 170° C. for 5 minutes to form a sheet, the sheet was excellent in rigidity and strength and exhibited cloudiness indicative of increase in crystallinity.
Patent Document 4 describes lactic acid or lactic acid oligomers are useful as a plasticizer of polylactic acid to lower the glass transition temperature and to impart flexibility.
Patent Document 5 describes lactate salts and benzoate salts as nucleating agents to blend in a biodegradable composition containing polylactic acid. According to Examples therein, when 1% of calcium lactate was blended with a polylactide copolymer and the blend was injection-molded with a die kept at about 85° C. for a retention time of 2 minutes, the resultant molding had insufficient crystallinity and hence it was further annealed in a die at about 110 to about 135° C.
According to Patent Document 6, however, a lactic acid-based polymer was indeed blended with a conventional nucleating agent, such as talc, silica, and calcium lactate, and it was tried being injection-molded, but crystallization was slow and the molding was brittle, providing no moldings durable in practical use. The document describes, therefore, that the lactic acid-based polymer has limitation on its use because, even if it is blended with conventional talc, silica, calcium lactate, or the like and molded by a common process such as injection molding, blow molding, and compression molding, the crystallization is slow, and resultant moldings have a service heat resistance as low as 100° C. or lower and unsatisfactory impact resistance.
Patent Document 7 describes that, when polyglycolic acid and/or derivatives thereof was added to poly-L-lactide or others as a nucleating agent, the crystallization rate increased, the cycling time in injection molding was, therefore, shortened, and moldings with excellent mechanical properties were obtained. Furthermore, the document describes that in injection molding, the values of crystallinity at a cooling time of 60 seconds was 22.6% with no nucleating agent added and 45.5% with the nucleating agent added. Patent Document 6, however, describes that when injection molding of the lactic acid-based polymer was indeed tried without any nucleating agent, molding was failed under conditions where the die temperature was not lower than the glass transition temperature as described in Patent Document 7.
Patent Document 8 proposes addition of stabilizers to a polylactide mixture in an amount effective for reducing the ratio of depolymerization at temperatures not lower than the glass transition temperature, wherein the stabilizers include antioxidants, dehydrating agents, drying agents, and catalyst deactivators. The catalyst deactivators listed there include alkylhydrazines, arylhydrazines, amides, cyclic amides, hydrazones, acylhydrazides, diacylated hydrazine derivatives, and heterocyclic compounds, among which, bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazide is listed as a preferred catalyst deactivator. Addition of bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl]hydrazide was able to suppress depolymerization in a molten state, but not able to provide any resin composition that was excellent in crystallinity and transparency and capable to be processed with a molding cycle similar to that for common resins. That is, the deactivation of catalyst did not contribute to improving the molding cycle.
Patent Documents 9 to 11 describe the mixing polylactic acid or aliphatic polyesters with an aromatic or aliphatic carboxamide provided moldings with excellent crystallinity, transparency, and heat resistance. However, in practical processes including injection molding, such composition cannot be molded with a molding cycle similar to that for common resins, and hence its practical application is difficult.    Patent Document 1: Japanese Patent Laid-Open Publication No. S60-86156    Patent Document 2: Japanese Patent Laid-Open Publication No. H5-70696    Patent Document 3: Japanese Patent Application Laid-Open No. H5-504731 (WO 90/001521 pamphlet)    Patent Document 4: U.S. Pat. No. 5,180,765    Patent Document 5: Japanese Patent Application Laid-Open No. H6-504799    Patent Document 6: Japanese Patent Laid-Open Publication No. H8-193165    Patent Document 7: Japanese Patent Laid-Open Publication No. H4-220456    Patent Document 8: Japanese Patent Application Laid-Open No. H7-504939    Patent Document 9: Japanese Patent Laid-Open Publication No. H9-278991    Patent Document 10: Japanese Patent Laid-Open Publication No. H10-87975    Patent Document 11: Japanese Patent Laid-Open Publication No. H11-5849